Very low FPS on very good hardware

I’m getting very low FPS in any game mode, at any resolution with any settings, in “safe mode”, 32/64 bits, etc. Nothing works.

This is the only game that has this frame rate problem. Even in Deadliest Warrior I get 62 FPS.
Rocket League, Mirage and other games don’t have this issue.

An offline FFA in Arena with no bots gives me 40-50 fps. With 10 bots, I get 30 fps. I get 62 FPS on the main menu btw.

I tried lowering/maxing all graphic settings. Ragdolls and Decals at 0 have zero impact.

The PC is almost brand new and it should run the game just fine. I checked my hardware stats during gameplay. The GPU (RX 480) is plugged in to the monitor. CPU/VGA usage is okay (70-95%), temps are okay, clocks are okay.

On another much older PC (with a Phenom II 955 as a CPU and a 7850 as GPU) I get almost constant 62 fps. The only difference is that PC is running Windows 7 and this one is using Windows 10.

I’ve googled and other people have experienced the same issue with even better hardware and it all seems to point to Windows 10 PCs. Those who have reported this problem haven’t been able to fix their problems either.